The present invention relates generally to polishing devices for semiconductor wafers and more specifically relates to a roller constructed of a novel polyvinyl acetal composition with an abrasive outer skin surface containing metal or other abrasive particles allowing water or chemical flow through the roller body without clogging of the sponge pores thus precluding sponge material destruction while simultaneously providing an abrasive cleaning surface.
Many industries today require the ability to efficiently clean highly finished surfaces, removing particles and other surface contaminants. Specific articles having highly finished surfaces include, but are not limited to, wafers of semiconductor material, and memory disks.
In the manufacturing process of integrated circuits or semiconductor wafers the wafers are initially polished to planarize the surface prior to subsequent processing of depositing the desired circuits. A combination of chemical and mechanical polishing is generally used which presently requires the introduction of polishing chemicals onto the surface of the semiconductor wafer as it is being polished or cleaned by the roller or brush. The chemicals may contain fine abrasive particles such as silica (SiO2) and alumina (Al2O3). When the polishing is completed the semiconductor wafers must be cleaned to completely remove residual materials in order that the surface be made ready for subsequent photolithographic processing and other manufacturing steps.
Currently used cleaning brushes or rollers have their pores clogged with chemicals and abrasives and waste reducing flow and rapidly break down during the polishing process.
Devices for cleaning semiconductor wafers generally consist of a cylindrical roller or rollers passing over a wafer or polishing pads. The cylindrical roller conventionally includes tufted nylon or other types of bristles extending from a central core. The bristle roller brushes are fixed horizontally and rotate as the wafer or memory disk is passed between the bristle surfaced brushes while the surfaces receive large quantities of polishing solution and/or de-ionized water.
Wafers polished in this manner are often unacceptable due to uneven application of the bristles to the wafer surface. In addition, the softness of the bristles varies according to their composition, resulting in a heterogeneous mixture of bristles, each harder or softer than their counter parts. This results in breakage of the wafers as well as damage to wafer surfaces. Other drawbacks, resulting in unsatisfactory polishing, may be attributed to the hydrophobic nature of bristle tufted brushes. The fiber surfaces of these brushes are never wetted, and require large amounts of de-ionized water or polishing solution to work in the polishing process. Polishing devices using sponge material are generally more effective in polishing a surface but break down because of pore clogging and surface ripping. It has not been effective to clean such sponge brushes and they are generally discarded after an undesirable amount of residual polishing composition has been built up in the pores of the sponge. Since such sponge brushes are relatively costly, the sponge brushes are sometimes utilized beyond the point where they should be replaced resulting in inconsistent polishing of the wafers lowering the yield of semiconductor product wafers.
U.S. Pat. No. 4,098,728, discloses a polyvinyl acetal sponge and a method for making the same. In this method, pore spaces in the sponge are formed by gas bubbles; rather than pore forming chemical additives such as starch/sulfate combinations. Because the sponge disclosed in this patent does not have any starch residue, it has been particularly useful in medical applications in which starch residues can cause a foreign body reaction when in contact with human tissue.
The use of synthetic sponges, made of polyvinyl acetal for cleaning devices is well known. For example, U.S. Pat. No. 4,566,911 discloses a roller scrubbing device using a polyvinyl acetal material for cleaning semiconductor chips having a surface layer of elastic polyvinyl acetal material with an average pore diameter ranging from 10 to 200 microns. U.S. Pat. No. 6,080,092 discloses an industrial sponge roller device with a cylindrical body of polyvinyl acetal material and a plurality of projections of a truncated conical shape extending from an outer surface of the cylindrical body.
U.S. Pat. No. RE 35,709 discloses using a low viscosity slurry comprising a polymerizable resin, abrasive particles and modifying particles to reduce the viscosity of the slurry for application to an article.
Conventional synthetic sponges have a polymer structure with xe2x80x9cdead end pocketsxe2x80x9d formed therein that trap residue and trace amounts of metals and have non-uniform pore sizes causing fluid backup and residue deposit. As the sponge wears, these metals can come out of the sponge in the form of particulate matter. Such particulate matter can damage the surfaces that are to be cleaned. Further, this type of sponge has tiny fibrils in the pores thereof that are a result of spaces between the pore forming chemical additive during a cross-linking reaction. xe2x80x9cCross-linkingxe2x80x9d is the formation of ester bonding between chains of the two adjacent hydroxyl groups that occur with the reaction of polyvinyl alcohol and aldehyde. This reaction hardens and strengthens the resulting material.
Semiconductor wafers must be polished prior to any processing steps used to produce semiconductor devices. Due to the fragile nature of semiconductor wafers, achieving a high degree of polish, as well as a high yield of wafers is difficult. Many polishing devices produce a low yield of polished wafers due to breakage or an unacceptable level of surface contaminants. Memory discs provide less of a problem with breakage, being more durable.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
A polishing roller of polyvinyl acetal having a uniform small pore size throughout the material in which over 80% of the pores of the material range from 7-40 microns in diameter with a mean flow pore diameter of about 20 microns and a mean flow pore pressure of 0.334 PSI. An abrasive such as SiO2, Al2O3 and the like is applied to the outer surface of roller using a low viscosity slurry as the matrix and absorbed by the pores of the foam to provide an abrasive thin skinned polishing surface.
Accordingly, it is an object of the present invention to provide a new and improved polyvinyl acetal sponge material for use in polishing semiconductor wafers which has greater durability than prior sponge material.
Another object of the present invention is to provide an improved even flow rate of water and chemicals from the center of the core of the roller to the outside abrasive skin of the sponge material for even application of same to the surfaces of the articles being polished without clogging and tearing of the roller surface.
It is another object of the invention to provide a material which allows better polishing of surface materials using no or reduced cleaning solution and uniform solvent delivery at low rates.
It is still another object of the present invention to provide an improved sponge material for polishing semiconductors that has better polishing and strength characteristics.
It is yet another object of the present invention to provide a sponge material capable polishing semiconductor surfaces and adding to the life usage of the sponge material used in polishing the semiconductor wafers as well as reducing the use of polishing abrasives and cleaning materials currently used in prior art roller devices.
The present invention solves the above noted problems with polishing sponges in a manner not disclosed in the known prior art.
In the accompanying drawings, there is shown an illustrative embodiment of the invention from which these and other objectives, novel features and advantages will be readily apparent.